Question

How I’m suppose to implement a macro that would do something like this:
//pseudocode

#define CHOOSE(X) if(X) expand to "" and if !X expand to return nullptr;

Edit

template<class SomePolicy>
struct M
{

template<class Policy = SomePolicy>
typename std::enable_if<IsThrow<Policy>::value,void>::type fnc()
{
}

template<class Policy = SomePolicy>
typename std::enable_if<!IsThrow<Policy>::value,std::nullptr_t>::type fnc()
{
    return nullptr;
}
};  

@All_WHO_TRIED_TO_HELP_ME Hey Guys I want to thank you very much for your priceless help. It couldn’t be done without you. Thanks to all of you. Thank you.

Answer 1

Answer to edit section.
Use the fact that expression return g(); is valid even if g return void.

template <class T> T GetDefault() { return T(); }
template <> void GetDefault<void>() {  } // special case for void

and use

return GetDefault<void>();

or

return GetDefault<std::nullptr_t>();